Sheets of material are often used in various industries and in a variety of ways. These materials can include paper, plastic, and other materials manufactured or processed in webs or sheets. As a particular example, long sheets of paper or other materials can be manufactured and collected in reels. These sheets of material are often manufactured or processed at a high rate of speed, such as speeds up to one hundred kilometers per hour or more.
It is often necessary or desirable to measure one or more properties of a sheet of material as the sheet is being manufactured or processed. For example, in a paper sheet-making process, it is often desirable to measure the properties of the sheet (such as its basis weight, moisture, color, or caliper/thickness) to verify whether the sheet is within certain specifications. Adjustments can then be made to the sheet-making process to ensure the sheet properties are within the desired range(s).
Many optical and image-based measurements involving a sheet of material often require the sheet to be confined in a specific position or plane. For example, there is often a narrow range of working distances (from a sensor to the sheet) and/or a narrow range of tilt angles (with respect to illumination or examination of the sheet) that provide proper measurements with these techniques. Deviations from the expected or required working distances, tilt angles, or other geometries may introduce bias, uncertainty, or other errors in the measurements. This problem becomes more pronounced when taking measurements of a moving sheet, which may flutter or otherwise move as it passes by or between sensors.
Existing solutions for constraining sheet position and sheet planarity are often of limited use. For example, existing solutions could stabilize a sheet for one sensor while disturbing the sheet near other sensors. Also, stationary contacting devices (such as caliper buttons) can apply friction to the sheet, which can cause unwanted markings on the sheet, increase the risk of a sheet break, and are difficult to set up (since contact pressure may be grade-dependent). Further, aerodynamic devices (such as a backstep coanda or helical vortex) often do not guarantee good sheet position or sheet planarity since, for example, sheet position may be unstable in time and can vary with sheet tension. In addition, non-stationary contacting devices (such as guide rollers) often cannot guarantee good sheet position or sheet planarity since there is a substantial boundary layer of air moving with the sheet, typically resulting in overpressure where the sheet attaches to a guide roller and underpressure where the sheet detaches from a guide roller. This can cause deflection of the sheet path between guide rollers, affecting the sheet's level and tilt and leading to sheet position instability. Moreover, overpressure, underpressure, and turbulence can vary with speed, sheet tension, and permeability (very low permeability sheets may actually “float” over the guide rollers without coming into non-slip contact with the guide rollers).